1. Field of the Invention
The present invention relates to the detection of the state of charge of a battery assembly, and especially relates to a method and device by which the state of charge can accurately be detected, even if the variation in charged amount among batteries composing a battery assembly becomes large. Furthermore, the present invention relates to a device by which said variation in charged amount can easily be detected.
2. Description of the Related Art
A typical electric motor car in which an electric motor provides the driving force has a secondary battery (hereafter referred to simply as a battery), and the electric motor is driven by the electric power charged in this battery. Here, a hybrid powered automobile which is a type of electric motor car, will be described.
A hybrid powered automobile has an electric motor and a combustion engine, and further includes an electric generator. The electric generator generates electric power by being driven by the combustion engine, and the battery is charged by the thus generated electric power. Furthermore, the battery is also charged during regenerative braking. There may be a case where an electric generator also serves as an electric motor (motor-generator). In a hybrid powered automobile, the charged amount of a battery can freely be controlled by shifting between charge and discharge.
For example, when slowing down on a downhill road or the like, the charged amount is raised by regenerative braking. The combustion engine is then stopped and the vehicle runs by the output power of the electric motor. Thus, by utilizing the electric power obtained by the regenerative braking, a high energy efficiency can be achieved. When it is found that the charged amount is lowering, the electric generator is driven by the output power of the combustion engine, and the generated electric power is charged in the battery.
For the control of the charged amount, previously, SOC (state of charge) has been used as a quantity expressing the charged amount. SOC is the ratio of the occasional charged amount to the charged amount in the state of full charge. SOC is 100% in the state of full charge, and 0% in the state of no charge. It is desirable that SOC be controlled to be a value towards the middle of 100% to 0%, for example, approximately 50 to 60%. If SOC is controlled to be such a value, the battery can sufficiently take in the electric power generated during the regenerative braking, while still being able to immediately supply sufficient electric power to the electric motor according to the request.
Furthermore, in an electric motor car, usually, the upper limit value and the lower limit value of the charged amount are determined. The upper limit value is set considering the charging efficiency, the heat evolution of a battery, and the like. Furthermore, the lower limit value is set considering the electric power necessary during the starting up of the combustion engine, and the like. The charged amount is controlled so as not to exceed either of the upper limit value or the lower limit value.
In an electric motor car, in order to obtain a high voltage, a battery assembly having a number of batteries connected in series is commonly used. The temperature of the batteries composing a battery assembly is not equal, and, especially in the service environment of an automobile or the like, differences in temperatures among batteries arise fairly easily. Furthermore, the full capacity is different for each battery, and the charging efficiency (ratio of the amount of the current to be charged, to the amount of the supplied current) is also different. Therefore, there is a variation in the actual charged amount (rest charged amount, rest capacity) of each battery composing a battery assembly.
Therefore, as shown in FIG. 1, it is arranged to perform control so that the charged amount of any battery composing a battery assembly will not exceed either the upper limit value or the lower limit value. At the upper portion of FIG. 1, the shaded area m shows the range of the variation in charged amount. The charged amount of a battery increases and decreases while keeping the state of the variation. In other words, the area m in FIG. 1 moves leftward and rightward as it is. In order to prevent over discharge and over charge, the charged amount is controlled so that the area m will not protrude out of the upper limit value or the lower limit value. Generally, if the charged amount of any one battery has reached the upper limit value or the lower limit value, respective additional charge or discharge is restrained.
However, the variation in charged amount changes with the elapse of the operating time of a battery, and generally gradually enlarges. Previously, there has been such a problem that as will be described more fully below, that the detection of the charged amount is not properly performed and the control of charge is not properly performed since the change of the variation is not considered.
At the lower portion in FIG. 1, the area m' is the range of the variation in charged amount after being enlarged. It is assumed that charging is performed beginning from the state where area m' is positioned at the left end as shown in the figure. Since the enlargement of the variation is not considered, the charge is controlled on the assumption that the area m' can move rightward by a distance equal to that before the enlargement of the variation. However, this assumption that the area m' can move to the position shown by the dotted line is not correct. Actually, as shown in the figure, the area m' reaches the upper limit value considerably earlier than the expected time. At this time, a rapid shift from charge to discharge is necessary.
Such control negatively effects drivability. In a hybrid powered automobile, the distribution of the driving force between an electric motor and a combustion engine is controlled so that charge and discharge may properly be performed while tracking the amount of charge. If a shift from charge to discharge is suddenly performed, the distribution of the driving force also suddenly shifts, causing the driving force to shift in stages and giving the occupants an uncomfortable feeling. For example, when an active charge is suddenly shifted to discharge, the combustion engine driving the electric generator stops and the motor-generator is shifted from generating mode to driving mode. These problems also arise similarly during the discharge. That is, the occupants may feel discomfort because of a sudden shift from discharge to charge.
These problems arise because the charged amount is not accurately detected. As an example, as shown at the upper portion in FIG. 1, the center of the area m is set to be the representative value X of the charged amount. The position of the representative value X is found by adding one half of the width of the variation (D/2) to the lower limit value of the charged amount when the area m is positioned to the left. However, if the change in variation is not considered, the representative value X is shifted from the center Y of the actual area m' as shown in the lower portion in FIG. 1. Consequently, value X used to represent the charged amount becomes incorrect.
The control target value of the charged amount may be set to, for example, approximately SOC 60%. If the representative value X is wrong, this control target value is not achieved because the center Y (real representative value) exists at different position when the representative value X is SOC 60%.
When the variation becomes large, a further problem arises. At the lower portion of FIG. 1, when the area m' has reached the upper limit value of the charged amount, the representative value X has yet to reach the control target value. Even so, discharge becomes necessary so as to avoid overcharge. This results in that a contradiction arises in control and a suitable control cannot be performed. This phenomenon may arise not only on the charge side but also on the discharge side. In such a case, discharge and charge are repeated without achieving the control target, and consequently, repeated switching between charge and discharge may arise.
As descried above by using a concrete example, if the variation changes, the charged amount is not properly detected, and the control of charge may not properly be performed. In order to avoid such a problem, it can also be considered that the variation theoretically is large. However, approaching the problem in this way results in the upper or lower limits of the charged amount being wastefully limited and in the under utilization of battery performance.
Furthermore, as a related art, there is a battery rest capacity display device described in Japanese Patent Laid-Open Publication No. Hei 8-163705. In said publication, the state of charge is found considering the degradation of a battery. However, the variation in the charged amount of a battery is not considered, and the detected allowance of discharge is not accurate.
Especially, problems such as described above are obvious in a battery having a charging characteristic similar to that of a nickel hydrogen battery (NiMH). IV judgment is a well known SOC detecting system. In IV judgment, SOC is found by using the fact that there is a correlation between the voltage and current of a battery and SOC. However, a nickel hydrogen battery has such a charging characteristic as shown in FIG. 2. The value of the voltage relative to SOC is approximately constant. The voltage largely changes relative to SOC only in the area where SOC is nearly 100% and in the area where SOC is nearly 0%. IV judgment is difficult in the area where the voltage is constant, and the area where the IV judgment can suitably be performed, is limited to the areas of both ends.
Accordingly, IV judgment cannot be used except when the charged amount is the upper limit value, the lower limit value, or a value near one of those values. Because of this limitation, methods wherein the change of the charged amount is traced by integrating the value of the current flowing through a battery are considered. When referring to FIG. 1, the movement of the area m with slanting lines is traced and, consequently, the charged amount is detected. However, during this tracing of the charged amount, as described above, the upper limit value or the lower limit value of the charged amount may be instantly detected by IV judgment. Furthermore, in the representative value X for the tracing of the charged amount, an error according to the change (enlargement) of the variation may arise.
On the other hand, in the above description, it has been explained that there is a variation in charged amount of a battery composing a battery assembly. If the charged amount of each battery is detected, the variation in amount of charge amount can also be found. However, a nickel hydrogen battery has a charging characteristic of FIG. 2. While the battery is used, the amount of charge exists in the state of being scattered in a part where the voltage is constant. Accordingly, the amount of charge cannot be detected by IV judgment. Therefore, for a nickel hydrogen or similar battery having a charging characteristic as shown in FIG. 2, it has been difficult to detect the variation in charged amount while the battery is used.